1. Field of the Invention
This invention relates generally to the ion implantation systems and methods employed in the fabrication process for manufacturing semiconductor devices. More particularly, this invention relates to apparatus for effecting the ion beam neutralization for ion implantation and, more particularly, relates to an apparatus for neutralizing a positively charged ion beam by providing low energy electrons from a sufficiently cooled plasma flood source.
2. Description of the Prior Art
Electrical charge buildup on a wafer surface when ion beams are applied for implantation is still a technical difficulty faced by the semiconductor industries. Ion implantation has been used as a standard way to dope impurities, such as boron, phosphorus, and arsenic, into semiconductor materials for more than two decades. Its property of controllable doping direction makes it irreplaceable by other doping techniques in manufacturing sub-micron integrated circuits. An ion implanter used for ion implantation purpose usually includes an ion source, ion beam transport optics, and a process chamber where ion implantation occurs. An electron or plasma flood source is always an important component of an implanter. Not only can it help to reduce charge build-up on target wafers, but also it can help to increase ion beam transportation efficiency to the target wafers, especially for low-energy ion beams.
It is well known in the art of integrated circuit (IC) manufacture that a charged ion beam will produce a buildup of charge on the surface of the semiconductor target. This charge may not be removed from the surface of insulating and semi-conductive wafer material. In this situation, positive charge builds up on the surface of the material since most ion beams are positively charged. Such charge may interfere automatic wafer handling due to sticking, may break through layers of micro-circuitry, and may affect implant uniformity due to charged portions of the wafer surface deflecting the ion beam. The presence of such surface charges therefore is believed to reduce yields in the production of integrated circuits.
One effective way to reduce positive charge build-up on wafer surface is to supply similar amount of negative charges, e.g. electrons, on the wafer surface. Electron or plasma flood sources, or alternatively called electron or plasma shower sources, are usually used to supply these electrons and introduce them onto the wafer as the ion beam strikes the wafer surface. As the requirement of implantation expands towards low energy ions with large current, e.g. for shallow junction formation, electron and plasma floods were also used to reduce space charge blow-up and to enhance the transportation of low energy ion beams with large currents.
To achieve the dual purposes of reducing wafer charging and space charge blow-up, electrons are directed towards the passing ion beam instead towards the target wafers, since the direct application of electrons to the wafers can produce contamination from the filament of the electron source. The direct radiation from the filament can cause wafer heating and non-uniformity across the wafer during implantation. The primary electrons from the flood source with high energies, e.g. 70 eV, can also induce high negative potential on wafer surface and may damage the integrated circuits.
For above reasons, the electrons are introduced in the beam generally transverse to the direction of beam projection to produce a neutralization of the beam. The individual ions in the beam are not neutralized since the combination cross-section of an ion and an electron is small. A neutralization of a beam means an effective neutralization of charge within the volume of the beam. The electrons in the neighborhood of the ion beam are attracted by the positive beam potential and travel together with the ion beam. However, with this approach the efficiency of entrapment of electrons within the beam may be low due to the high velocity of the primary electrons emitting from the filament and to the low capture cross-section of the electrons by the beam. Generating electrons with low energies would increase the probability of entrapment.
There are several ways to produce low energy electrons. One is to generate these electrons outside the primary electron source. In this case, a dummy target, usually made of aluminum or graphite, is located on the other side of the ion beam opposite to the electron source. The primary electrons with energies about 70 eV are extracted and strike the target, generating large amount of secondary electrons. Most of these secondary electrons have much lower energies, e.g. less than 10 eV.sup.1, and easier to be trapped with the ion beam. This type of electron flood source has been described in many patents, e.g., D. A. Robertson et al., "Apparatus for enhanced neutralization of positively charged ion beam", U.S. Pat. No. 4,463,255, 1984, V. M. Benveniste, "Ion beam neutralization means generating diffuse secondary emission electron shower", U.S. Pat. No. 5,164,599, 1992, and J. D. Bernstein et al., "Biased and serrated extension tube for ion implanter electron shower", U.S. Pat. No. 5,903,009, 1999. There are several technical difficulties in applying this electron source for ion implantation. Specifically, the primary electrons, through elastic collisions with other electrons, may drift to wafers and produce high electric fields on wafer surfaces thus induces micro-circuitry damage. For effective neutralization, the dummy target has to be close to the ion beam. The patented electron source increases the opportunity for the ion beam to strike the dummy target during beam tuning and create metal particles from the dummy target surface. These particles will drift to the processed wafers and induce metal contamination on wafer surfaces.
 FNT .sup.1 Ion Implantation Science and Technology, edited by J. F. Ziegler, p. 576-582, 1996.
The second way to generate low energy electrons is to create them inside the primary electron source. An ionizable gas is introduced into the source. The primary electrons impact the gas atoms, knock off one or more electrons from each atom, and generate plasma in which positive charges (atomic ions) and negative charges (electrons) are almost equal. The electron energies inside the plasma depend on the plasma temperature, and are usually less than 5 eV with narrow distribution. These low energy electrons together with ions, or the plasma, would drift out of the flood source and reach the ion beam, as illustrated in FIG. 1 below in the section of "Detail Description of Preferred Embodiment", therefore this type of source is called plasma flood source. Plasma flood sources have been described in many patents, e.g., H. Ito et al., "Plasma flood system for the reduction of charging of wafers during ion implantation", U.S. Pat. No. 5,399,871, 1995. The possibility to trap the electrons in the ion beam is inverse proportional to the electron velocities. Since the fast moving electrons are dragged by the slowly moving heavy ions from the flood source, the electrons have more time to stay around the ion beam and easier to the trapped in the ion beam. When more ions in the ion beam are neutralized after trapping greater number of electrons, its net beam potential decreases, thus reduces the number of trapped electrons. Therefore, this type of electron flood source can self-regulate the amount of trapped electrons to avoid over or under compensating the positive beam current. Since the electrons from the source have low energy, the wafer damage caused by high-energy electrons can be avoided. There is no dummy target required, and the flood source can be placed far away from the ion beam, therefore there is minimum chance that the ion beam will strike any metal surface near the wafers except the process chamber. However, the discharge inside the flood source can still generate metallic particles such as tungsten, aluminum, or molybdenum, depending the building materials of the flood source housing. These metallic particles can drift out of the source and deposition onto the wafer surfaces.
Reducing metal contamination is important for increasing the yield of integrated circuit manufacturing. A person of ordinary skill in the art still faces the demand of providing an improved flood source that induces much less metal contamination while keeps most advantages of other conventional flood sources.